1. Field of the Invention
The present invention relates to apparatus and methods for electron beam imaging.
2. Description of the Background Art
The two most common types of electron microscopes available commercially are the scanning electron microscope (SEM) and the transmission electron microscope (TEM). In an SEM, the specimen is scanned with a focused beam of electrons which produce secondary and/or backscattered electrons as the beam hits the specimen. These are detected and typically converted into an image of the surface of the specimen. Specimens in a TEM are examined by passing the electron beam through them, revealing more information of the internal structure of specimens.
Bright field imaging and dark field imaging are often used in the context of TEMs. A bright field image may be formed in a TEM by selecting electrons from a central diffraction spot to form the image. A dark field image may be formed in a TEM by selecting some or all of the (non-central) diffracted electrons to form the image. The selection of electrons may be implemented using an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible through the aperture.
Dark field imaging are typically less commonly used in SEMs. SEM dark-field detection systems may be categorized as below-the-lens detectors or behind-the-lens detectors.
A conventional SEM dark field detection system with a below-the-lens configuration 100 is depicted in FIG. 1. In a below-the-lens configuration 100, the detectors 104 are positioned below the objective lens 102 at the bottom of the electron beam column (near the specimen). Unfortunately, immersion objective lens technology interferes with the collection efficiency of below-the-lens detectors 104. Thus, non-immersion objective lens designs are typically used, resulting in higher lens aberration coefficients and leading to inferior image resolution. In addition, the polar angle discrimination threshold is not well controlled for such below-the-lens detectors 104 because the electron energy and emission azimuth can affect the polar angle acceptance of the detector 104. The definitions of the polar angle θ and azimuth angle φ in relation to the scattered electrons emitted from the specimen are shown by illustration in FIG. 2.
A schematic diagram of a conventional SEM dark field detection system with a behind-the-lens configuration 300 is depicted in FIG. 3. A typical behind-the-lens configuration 300 uses off-axis detectors 304 similar to those shown in FIG. 3. These detectors 304 are located “behind” the objective lens 302. In other words, the detectors 304 are located above and to the sides of the objective lens 302 in the electron beam column. The behind-the-lens configuration 300 allows the use of immersion objective lens technology, but unfortunately the behind-the-lens configuration 300 typically has poor polar angle discrimination. In addition, polar angle acceptance is sensitive to primary beam landing energy and beam scanning.
Another behind-the-lens approach uses a segmented detector in conjunction with a Wien filter. Immersion lens technology can be used with this approach, but the approach is sensitive to beam scanning and provides only poor azimuth discrimination and little or no polar angle discrimination.